Osteoarthritis of the hip affects over four million persons in the US and results in over 200,000 primary elective total hip replacements (THR) per year. Failure of the prosthesis is costly and disabling, resulting in 37,000 revision hip replacements annually in the US. Although increasing evidence points to a growing burden of revision total hip replacements over the next decade - with attendant complications, disability and costs - US population based rates of THR failure have not been determined. Further, there are no US population based studies of risk factors for THR failure. All data have been reported from referral centers, where a minority of THR's is performed. These observations highlight the urgent need for population-based studies of the rate of failure leading to revision THR and to identify risk factors, especially those that are modifiable. This proposal builds upon the extensive experience of the research team in population-based studies of the utilization and outcomes of THR in the US Medicare population. In previous work we assembled a claims-based cohort of 58,000 Medicare beneficiaries who had elective THR between July 1995 and June 1996. In this application we propose to follow this cohort for twelve years following the index THR to undertake the following specific aims: 1) To perform a nested case control study in order to identify factors associated with prosthesis failure leading to revision following elective primary THR. Risk factors will be ascertained from medical records, Medicare claims, Census data and American Hospital Association data. 2) To estimate the cumulative incidence rate of prosthesis failure leading to revision THR over twelve years in a national population-based cohort of Medicare beneficiaries, and to identify factors associated with prosthesis failure in the national claims cohort. The findings will provide a national estimate of the risk of THR failure and an assessment of factors associated with THR failure at a national level. We investigate risk factors at the patient, surgeon and hospital level, permitting inferences that will guide clinical and policy interventions at several levels. These data will permit clinicians to better inform patients about the anticipated longevity of their implants and about factors that may influence prosthesis survival, and consequently the patient's quality of life. Lay Summary: Total hip replacement is done frequently for severe arthritis of the hip. There is very little information available at a national level on how long the implants work properly until they cause pain and need to be replaced. This study uses information from Medicare claims and medical records to determine the risk of failure of the hip replacement and the factors that predict which patients will experience a failure.